Most current and nearly all past oil production methods leave as much as 50% of the original oil in place. Recent efforts to recover that oil remaining in the reservoirs have had considerable success. Among the more promising of the methods being used today is an enhanced oil recovery process referred to as a surfactant flood. This chemical method has brought with it some very difficult oil dehydration problems. These problems are the direct result of the chemicals used in the reservoir to improve the movement of the oil out of the reservoir rock and into the producing wells. In the normal water-flood phase of producing oil from the reservoir, much of the oil in place remains because of the high interfacial tension between the oil and water. The droplets of oil will not flow freely through the capilaries in the reservoir rock. To overcome this mobility problem, producers are adding surface-active agents to the flooding fluids. These surface agents, called surfactants, reduce the interfacial tension between the oil and water. Stable micelles are formed between the oil and water and these micelles flow through the pores of the reservoir rock, moving the oil and water to the producing wells. The surfactant is the emulsifying agent and the presence of that surfactant in the produced fluids makes the subsequent oil and water separation very difficult in surface facilities. The very chemical features that produce success downhole makes the normal oil and water separation difficult. This is one of the problems that require new and different systems to accomplish the treatment of oil to make it salable and useful.
In field tests run by Shell Development Co. and reported in the December, 1977 Oil and Gas Journal, tracer response studies showed how much channeling of flow from the inlet to outlet occurs in the present separation devices used in oil and water separation processes. The hydraulic efficiency of large tanks used as oil and water separation devices proved to be very low, in the range of 5% or less. Because the separation of the oil and water is so difficult in the surfactant flood-produced fluids, good hydraulic efficiency is a must for the vessels used in the separation process. The system of the present invention offers a practical method of accomplishing oil and water separation in fluids produced by the surfactant flooding process. More broadly, the present invention enables the treatment of a large number of difficult-to-treat emulsions of secondary recovery, regardless of whether there is an element such as surfactant to be recovered and recycled.
Enhanced oil recovery processes, such as surfactant flooding, or fire flooding, that inject or generate chemicals that reduce the surface tension between oil and water, create rather difficult-to-treat emulsion problems. The surface tension reducers may be manufactured external to the reservoir and then injected; others may be a by-product of a reaction in the reservoir, such as fire flooding. From whatever source, these surface tension reducers cause a new set of problems not normally encountered in the on-surface processing of reservoir fluids. One pressing problem is that of the difficult-to-resolve emulsions both of the oil-in-water, and water-in-oil types. Another problem encountered is the wide variation in the volume and the stability of the emulsions produced over the life of the reservoir flooding operation.
When the tertiary operation first begins to respond to the new stimulus, the fluids produced may show an increase in uncontaminated oil that is relatively free of the emulsion-causing surfactants. As more and more of the reservoir fluids are produced, the character of the fluids coming from the stimulus of the surfactant action will be reflected in the emulsifying effects between the oil and water. During the early phase of the reservoir stimulus, the fluids are routinely prepared for sales and disposal with the conventional methods of oil and water separation and cleanup. As soon as the initial portion of uncontaminated oil is produced, the subsequently produced reservoir fluids become increasingly more difficult to prepare for sale and/or disposal, as the percentage of surfactant-containing fluids increase. The surfactant contamination varies as the surfactant front moves through the reservoir. As the surfactant front passes, the wells producing the fluid see an increase in surfactant contamination to an average maximum percentage equal to that which was injected into the reservoir when the flood was started. After the surfactant front passes the producing wells, the surfactant-laden fluids reduce as the flood progresses. The produced fluids again become more easily separated. A process for handling these reservoir fluids of varying characteristics should be designed to take advantage of these changes. The process system should be able to segregate the easy-to-treat oil and water from the difficult-to-treat oil and water.
In today's practice of enhanced oil recovery using chemical assistance, such as surfactants, the trend is to use surfactants that have saline tolerance. Most of the reservoirs which are candidates for use of surfactant-assisted enhanced oil recovery have been water-flooded for secondary oil recovery purposes. The connate water may have monovalent and divalent concentrations totalling over 100,000 parts/million. The water of injection that mixes with the surfactant may be to the order of 70,000 ppm. The new saline-tolerant surfactants are not influenced by salt levels of this magnitude. The oil-in-water emulsification takes place readily and the emulsion is not broken by the antagonistic effect of the monovalent or divalent ions in the salt water, as explained in Coffman U.S. Pat. No. 4,029,570. In these systems that use saline-tolerant surfactants, some other assistance, other than brine, must be used to coadunate the oil droplets in the oil-in-water emulsion. The oil droplets must be united before the separation process can begin. As related in the SPE paper No. 11985, presented by J. W. Ware in October, 1983, it was necessary to use 600 to 650 ppm of an additive generally referred to as a reverse-acting treating compound to assist in the coadunation of the dispersed small oil droplets. These so-called reverse-treating compounds are generally classed as water soluble polymers that are active at the oil-water interface to provide the antagonistic effect needed to cancel the effect of the surfactant. The water-soluble polymers provide the assistance needed to break the emulsions created by the saline-tolerant surfactants. This action is much like the action of concentrated brine described by Coffman supra and Newcombe, U.S. Pat. No. 4,216,079. Newcombe describes his laboratory experiments where he used very high concentrations of brines to obtain the monovalent and divalent ion antagonistic effect needed to resolve the oil-in-water emulsion.
In the present disclosure, the well fluids which have been emulsified by saline-tolerant surfactants, are concentrated into three portions: (1) a low-surfactant content oil that can be made salable with the conventional oil and water separation means, (2) a surfactant-rich oil and water emulsion in which the surfactant must be removed before the oil and water can be separated to make the oil salable, and (3) the third portion which is produced water with an oil remnant of perhaps 500 ppm or less which can be substantially removed by an efficient skimming process vessel. After the three portions are generated, the troublesome surfactant-rich portion can be given specific attention. The surfactant in the oil must be removed so that a separation of the oil and water can be accomplished. It is important to reduce the volume of troublesome surfactant-rich emulsion to the lowest value that can be accomplished in a practical manner, since the cosolvents that can dissolve the surfactant and remove it from the oil are not inexpensive, and the losses of the cosolvent in the separation operation are a function of the volume of surfactant-rich emulsion and its surfactant content. After the surfactant-rich emulsion portion of the well stream is concentrated, it is withdrawn from the concentration system and mixed with a cosolvent as taught in the ASTM Procedure D855 for "Analysis of Oil-Soluble Sodium Petroleum Sulfonates" to extract petroleum sulphates from oil to determine the equivalent weight of the sulfonate. This procedure demonstrates the ability of a cosolvent, such as diluent isopropanol, to selectively dissolve the surfactant from the oil of the surfactant-rich emulsion.